Resistance capacitance oscillator



A. W. STORM RESISTANCE CAPACITANCE OSCILLATOR Filed Nov. 21, 1946 L.mil, i: 7., 2 a! J April 18, 1950 iatenteci Apr. l8, 195

liFlCE RESISTANCE CAPACITANCE OSCILLATOR Andre Willem Storm, Sydney, NewSouth Wales, Australia, assignor to Hartford National Bank & TrustCompany, Hartford, Conn., as trustee Application November 21, 1946,Serial No. 711,431 In Australia June 19, 1945 Section 1, Public Law 690,August 8, 1946 Patent expires June 19, 1965 3 Claims.

In many types of resistance capacitance oscillators, the frequency ofthe generated voltage is determined by two circuits containingresistance and capacity components. It is common practice to make likecomponents in the two circuits, equal to each other. Hence, it isconvenient to use a two-gang variable condenser such as is employed inradio receivers for the capacity components so that the circuits may beadjusted simultaneously.

When such a two-gang variable condenser is used, the frame and rotor ofthe condenser must frequently be connected to the control grid of one ofthe valves in the oscillator, with one of the stator plates connected toearth and the other set of stator plates connected to a point which, sofar as alternating current potentials are concerned, is not at earthpotential. Hence, if any leakage occurs from the rotor or frame toearth, the leakage path is in parallel with that section of the gangcondenser whose stator is earthed.

The resistances used in such circuits are frequently of very high valueand the shunting efiect of this leakage from the rotor and frame of thecondenser to earth may materially affect the value of the resistance inthe circuit. As this leakage is a variable factor dependent upon manymatters such as atmospheric conditions, the stability and performance ofthe oscillator is adversely affected.

The primary object of this invention is to provide means wherebytheadverse effects of this leakage may be minimized.

In one practical form of the invention, the frame of the gang condenser(including, if desired, other components connected to the frame such asthe tuning control) is mounted on, but insulated from, a metal orelectrically conductive support such as a guard plate or screen whichis, in turn, insulated from the oscillator chassis or earth. The guardplate or screen is connected either to the cathode of the first valve ofthe oscillator or to a point in the oscillator circuit whose alternatingcurrent potential is approximately equal to and in phase with thepotential acquired by the frame of the gang condenser.

When the guard plate or screen is connected to the cathode of the firstvalve, the efiect of the leakage may be minimized by applying negativefeedback to the first stage of the oscillator in such a manner that theeffective input admittance of the oscillator, measured between thecontrol grid and cathode of the first valve, is materially decreased,thus reducing the adverse effects of the leakage as the resistance ofthe the condenser I.

2 leakage path, will, in effect be so much increased that the leakagewill not materially affect the performance of the oscillator.

When the guard plate or screen is connected to a point having analternating current potential equal to and in phase with that of theframe of the gang condenser, no difference of potential will existacross the leakage path between the frame of the gang condenser and theguard plate or screen and therefore the leakage will be ineffective.

The oscillator shown in the drawing comprises a first valve [0 havingcathode, grid and anode electrodes II, 12 and [3, respectively, and asecond valve 20 having cathode, grid and anode electrodes 2|, 22 and 23respectively. The anode i3 is energized from a source (not shown)through a load resistor l4 and is coupled to the grid 22 through ablocking capacitor [5. The input circuit of valve Ill comprises apositive feedback frequency determining network comprising a dualcapacity variable condenser I, a resistor 2 connected in series withcondenser i and a resistor 5 connected in shunt with the lower sectionof The rotor of condenser l is connected to the frame of the condenserand to the grid I 2. The input circuit of valve l0 also comprises anegative feedback network comprising series connected resistances 3 and4 the junction of which is connected to the cathode II. The said twonetworks are coupled to the anode 23 of valve 20 through a capacitor 25and the anode is energized through a load resistance 24 from a suitablesource (not shown) of anode potential. The circuit so far specificallydescribed conforms to standard practice and is found for example in U.S. Patent No. 2,268,872 issued to W. R. Hewlett on January 6, 1942.

If any leakage exists between the frame of the condenser and earth, itwill appear as a resistance 1 in parallel with the resistance 5 and willaffect the frequency of the voltage generated by the oscillator.Similarly any capacitance to earth from the frame of the condenser willalso affect the frequency of the generated voltage.

According to the invention, the condenser is mounted on a guard plate orscreen denoted by the numeral 6 which is connected to a point in thecircuit having a similar alternating current potential to the frame ofthe condenser. Leakage to the guard plate or screen 6 from the condenserframe will therefore have little effect.

In the drawing, the guard plate or screen is shown connected to thecathode of the first valve which has an impedance 4 connected in itscathode circuit. As is well known the presence of such an impedancecauses the cathode potential to follow the grid potential and thereforethis arrangement will decrease the effect of stray resistances andcapacitances from the frame of the condenser 'to" the guard plate oiscreen 6. Alternatively the guard plate or screen 6 may be connected tothe junction point of two resistors or impedances 3 and 4, which are sopro portioned that the junction point'is at approximately the samepotential with respect to earth for alternating currents as the -frameof the condenser.

Having now described my, invention, what 1 claim as new and desire tosecure by Letters Patent is:

1. A resistance capacitance oscillator compris ducting circuit,component, a feedback network,

coupled between the anode and control grid, meanscoupledto the feedbacknetwork to provide acircuitpointj havingapotential and phasesubstantially equal to. thepotential and phase of: the, control grid, anelectrically conducting. shield; member, interposed between andelectrically insulated from said frame member and said'circuitcomponent, and means to connect id: sh eld m b r to. said; i cu po '2. Aresistance, capacitanceoscillator comprising anv electrically conductingcircuit component, a discharge tube having a cathode, a control grid-andan anode, a feedback network coupled to, theganode, a frequencydetermining network. comprising a multiple section variable condenserhaving a frame member, two .rotor sections. and two stator sections,means. to connect one of said stator sections to said feedback network,means to connect the other of said stator sections to said circuitcomponent, means to connect said frame member and said rotor sections tothe control, grid, said. tr e e her being ioa qi ia id bit 'fi fiat isaid circuit component, a seriescircuit comprising two impedanceelements connected between said feedback network and said circuitcomponent, aneletrioally conducting shield member interposed etween: andelectrically insulated from said, framefiiiember and said circuitcompon'ntfand means to connect said shield memher to the junction, ofsaid impedance elements. 3. A resistance capacitance oscillatorcomprising an electrically conducting circuit component, a ms rgetub'ehaving a cathode, a control grid and an anode, a feedback networkcoupled to"'the'an ode, a frequency determining network comprising a.multiple section variable condenser having, asframamember, tworotorsections and two stator sections, means to, connect. one'pf. said;stator sections tofsaid feedback network, means.

to connect thev other? of 'saidistator sections to said circuitcomponent; means to. connect said:

frame member and said rotor sections to the control grid,said-framesmemberi being mounted on andfbeing I electrically insulated:from said feedbackunetworkuand saidi-circu'it-componentl'anelectrically. conducting shield member inter-posedbetween -and'electrically- "insulated front saidframe member and said circuitcomponent, and m'eal'ls t6 bdnnect'said shieldmenibrand-the cathode to'thejunction "ofsaid: 'iir'ipe 'd ance elements TQW- REFERENCES; CITEDThe following references are of recordinthe file 2? nines? A' N S NumberName Date 2,173,427 Scott Sept 19, 1939 2,268,872- Hewlett Jan. 6, 1942

